The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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Stock-Based Compensation - Additional Information (Detail) - shares	Mar. 31, 2018	Feb. 28, 2018	Dec. 31, 2017
Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding RSUs at March 31, 2018 (in shares)	293,000		259,000
Non-employee Director [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of options granted upon initial election to board (shares)	20,000	30,000
Additional number of options available for grant annually (shares)	15,000	20,000
Employees and Executives \| Restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Outstanding RSUs at March 31, 2018 (in shares)	162,000